1. Field of the Invention
The present invention relates to a process for cleaning up wastewaters by wet oxidation, where the wastewaters are produced in the preparation of hydroxyl and carbonyl compounds by the Hock process. In particular, the invention relates to a process for cleaning up wastewaters by wet oxidation, which wastewaters are produced in the preparation of phenol and acetone by the Hock process.
2. Description of The Background
The simultaneous preparation of phenol and acetone by the Hock process is based on the oxidation of cumene to cumene hydroperoxide and the subsequent acid-catalyzed cleavage of the cumene hydroperoxide to form phenol and acetone. The catalyst in this reaction is preferably sulfuric acid. Depending on the process procedure, the production of phenol and acetone is accompanied by formation of a varying amount of byproduct. The different wastewater streams which are generally produced in the process, in particular due to various washing processes in the course of the workup of the cleavage product, are therefore generally contaminated with various organic and/or inorganic substances at varying concentrations. The same generally also applies to wastewaters from other Hock processes. These sometimes acidic, sometimes basic, wastewaters, generally require, either together or separately, a pretreatment to reduce the wastewater loading prior to introduction into a biological effluent treatment plant, in order not to overload the biological purification stage and to decrease the content of poorly biodegradable constituents. In particular, phenol-containing wastewaters, because of the toxicity of phenol, cannot be discharged into open bodies of water.
To clean up phenol-containing wastewaters from the Hock process, extractive processes using, e.g. cumene or acetophenone as extraction medium (e.g. DE 1 075 119 and DE 22 35 493), steam distillation processes and adsorption to surface-active substances, such as activated carbon or synthetic resin ion exchangers are known (cf. Ullmann's Encyclopedia G' Industrial Chemistry, 5th edition, Vol. A19, p. 308, VCH Verlagsgesellschaft, Weinheim, 1991). A residual phenol content is obtained which can be removed in a biological effluent treatment plant.
However, recently, still lower wastewater loadings than are achievable using these processes alone, which processes are orientated toward phenol, are increasingly being sought after. A wastewater loading as low as possible with respect to the entire spectrum of possible constituents is desired not least for environmental reasons. In addition, government regulations may impose increased costs for the treatment of wastewaters with high loading, which also stimulates the need for process which reduce wastewater loadings.
A generally known process of chemical wastewater treatment for improving the biodegradability of wastewater constituents and for reducing the wastewater loading is wet oxidation. Wet oxidation is a process in which, at elevated temperature and an elevated pressure to maintain the liquid phase, wastewater streams whose constituents are degraded too slowly in the course of the biological wastewater purification, or which can adversely affect this degradation, are worked up. The oxidizing agents used are preferably air or oxygen. In the ideal case, hydrocarbons for example, in the wet oxidation produce solely carbon dioxide and water. A gross differentiation is made between low-pressure (120.degree. C., 6 bar) and high-pressure processes (320.degree. C., 200 bar) (Rompp-Chemie-Lexikon [Rompp's Chemistry Lexicon], 9th edition, 1991). Although there is a general trend for higher temperatures to cause higher degradation rates in wet oxidation, the degradation rates achievable are always specific to a substance (see, for example, B. Perkow, Steiner. Vollmuller: Na.beta.oxidation-Ein Beitrag zum Stand der Technik [Wet oxidation], Chem.-Ing.-Tech. 52 (1980), No. 12, p. 943 ff.). Therefore, various wet oxidation processes have been developed for specific applications, such as for wastewaters from the cyclic condensation of acetone (DE 44 28 413 A1) or for cyanide-containing wastewaters (e.g. DE 44 10 04 A1). The use of catalysts in wet oxidation to decrease the temperature required is also disclosed (e.g. DE 196 05 421 A1).
JP 1-310794 discloses a wet oxidation process preferably suitable for cleaning up wastewaters from the Hock process: the wastewaters containing organic substances are adjusted to a pH of from 1-3, preferably with sulfuric acid, nitric acid or hydrochloric acid, and then treated with an oxygen-containing gas at high temperatures and pressures in the liquid phase without a catalyst. Preferably, the treatment is carried out at from 175-200.degree. C. and apparently superatmospheric pressures between approximately 3 and 6 bar, based on the vapor pressure, i.e. at pressures between approximately 12 and 22 bar absolute, for from 2-6 hours. The oxygen-containing gas can be pure oxygen or air for example.
However, the need remains for improved processes which overcome the disadvantages described above.